You have a UNIQUE index on username. You want to update a row's username to a value that already exists in another row. What happens?

hard📝 Application Q9 of 15

SQL - Indexes and Query Performance

You have a UNIQUE index on username. You want to update a row's username to a value that already exists in another row. What happens?

AThe update sets the username to NULL automatically

BThe update succeeds and duplicates the username

CThe update fails due to UNIQUE constraint violation

DThe update deletes the other row with the duplicate username

Step-by-Step Solution

Solution:

Step 1: Understand UNIQUE index effect on updates
Updating a UNIQUE indexed column to a duplicate value violates the constraint and causes failure.
Step 2: Analyze the update operation
The database rejects the update to prevent duplicate usernames.
Final Answer:
The update fails due to UNIQUE constraint violation -> Option C
Quick Check:
UNIQUE index blocks duplicate updates [OK]

Quick Trick: Updating to duplicate UNIQUE value causes error [OK]

Common Mistakes:

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